1. Field of the Invention
The present invention relates to the formation of components for disk drives having graded properties. Particularly, the invention provides a method for forming a metal matrix composite article having graded properties. The graded properties are achieved by, for example, locating differing amounts of filler material in different portions of a formed article and/or locating different compositions of filler material in different portions of a formed article. In addition, the invention provides for the formation of macrocomposite bodies wherein, for example, an excess of matrix metal can be integrally bonded or attached to a graded metal matrix composite portion of a macrocomposite body.
2. Description of the Background Art
Current trends in the computer industry are toward lighter, smaller, more reliable computers with the capability of higher data storage and faster storage and retrieval of information. To this end, computer components, such as those for hard disk drives, are preferably lighter, stiffer, damped, smaller, more reliable, and able to perform at the faster speeds and high track densities desired. To meet these qualifications, the computer components and the conventional materials forming those components will require improvements in physical and performance properties.
Components for hard disk drives include devices such as sliders, load beams, E-blocks, actuator pivot bearings, disks, spacers, clamps, spindles, ball bearings, thrust bearings, journal bearings, base plates, housings, and covers. A slider typically carries a read/write head for reading and writing information on the disk itself. These read/write heads are transducers which read and write data onto the magnetic hard disk. Each slider in a disk drive is attached to a suspension assembly, which typically includes a flexural element attached to a load beam. Varying or uncontrolled slider/disk gap clearance can degrade the read/write signal reliability, which reduces the quality of the disk drive. Similarly, varying or uncontrolled read/write head to data track positioning can degrade the read/write signal reliability, which also reduces the quality of the disk drive. Thus, the design and materials used in these components are quite important.
The interest in composite products comprising a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like, has arisen because metal matrix composites combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. Metal matrix composites reinforced with ceramics such as silicon carbide in particulate or platelet form are of interest because of their higher stiffness, wear resistance and high temperature strength relative to metal.
Metal reinforced or ceramic reinforced metal matrix composites (MMC) are composed of a low density metal matrix surrounding reinforcing particles of low density and high specific stiffness. These can be used to manufacture actuators. Preferred low melting temperature metals to be used for the matrix are alloys of aluminum, magnesium and zinc. The reinforcing materials might be either a high melting point metal or a ceramic. The high melting point reinforcing metals of interest in MMCs are boron and beryllium. Ceramics of interest are silicon carbide (SiC) aluminum nitride (AIN), beryllia (BeO), silicon nitride (Si.sub.3 N.sub.4), alumina (Al.sub.2 O.sub.3), titanium carbide (TiC), magnesia (M.sub.3 O), titanium boride (TiB.sub.2) or boron carbide B.sub.4 C). Other reinforcements of interest are silicon and carbon fiber. These reinforcing materials have several advantages: for example, the specific stiffness of Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, SiC, B.sub.4 C and beryllium are approximately 6 times greater than that of 6061 aluminum, the current comb extrusion alloy. The damping properties of MMC are usually better than 6061 aluminum because of the internal particle to metal interfaces which help to absorb and scatter vibrational energy.
Furthermore, alumina and silicon carbide have low electrical conductivity and may result in electrostatic charges which are not easily discharged. Therefore, data destruction due to a buildup of static electric charges can occur on pure ceramic E-blocks.
Various metallurgical processes have been described in the art for the fabrication of aluminum matrix composites, and liquid-metal infiltration techniques which make use of pressure casting, vacuum casting, stirring, and wetting agents. With powder metallurgy techniques, the metal in the form of a powder and the reinforcing material in the form of a powder, whiskers, chopped fibers, etc., are mixed and then either cold-pressed and sintered, or hot-pressed. The maximum ceramic volume fraction in silicon carbide reinforced aluminum matrix composites produced by this method has been reported to be about 25 volume percent in the case of whiskers.
The production of metal matrix composites by powder metallurgy techniques utilizing conventional processes imposes certain limitations with respect to the characteristics of the products attainable. The volume fraction of the ceramic phase in the composite is limited typically. Also, the pressing operation poses a limit on the practical size attainable. Only relatively simple product shapes are possible without subsequent processing (e.g., forming or machining) or without resorting to complex presses. Also, non-uniform shrinkage during sintering can occur, as well as non-uniformity of microstructure due to segregation in the compacts and grain growth.
Now popularly referred to as semi-solid metal processing (SSM), there are three commercial embodiments that are finding increasing use, a) semi-solid forging, b) semi-solid billet die casting and c) Thixomolding.RTM. by Thixomat, Inc. The lower temperatures and higher apparent viscosities of thixotropic SSM slurries, as compared to superheated liquid metals, provide demonstrable advantages especially upon introduction into a die cavity during injection molding. The thixotropic characteristics of metal alloys result from a fundamental alteration of the microstructure caused by mechanical shearing of the dendrites of a semi-solid slurry. This altered physical state consists of spheroidal particles that have been created by degenerating the primary dendrites and suspending them in a continuous liquid matrix. It exhibits non-Newtonian viscous behavior (1994 Ghosh, et al.). Consequently, it is possible to produce finished net shape parts with improved mechanical properties and reduced porosity when compared to die casting.
In U.S. Pat. No. 5,672,435 a multiphase ceramic-material is used to make disk drive components by using metal infiltration of a porous ceramic body. Powdered metal and ceramic precursors are reacted under heat and pressure to create ceramic composites. The brittle property of both of these materials makes their use in disk drive E-blocks problematic for arm height adjustment and swaging of suspensions onto arm tips.
Semi-solid processing of beryllium-aluminum alloys to make actuator E-blocks is disclosed in U.S. Pat. No. 5,551,997. The processing temperature is held below the melting point of beryllium and above the melting point of aluminum so that the solid beryllium is surrounded by molten aluminum. Forming complicated geometries by semi-solid beryllium-aluminum has been found to generally be limited to 30-40% by weight of beryllium.
In U.S. Pat. No. 5,260,847 an E-block made beryllium-aluminum is made first by extruding an oversized, general outer form, and then machining all of the dimensions. This process is not economically efficient because of the large amount of expensive waste material containing a high percentage of beryllium and the additional expense of machining every dimension out of an alloy that is known to be more difficult to machine that aluminum.
Using investment casting of beryllium-aluminum alloys, disclosed in U.S. Pat. Nos. 5,642,773 and 5,578,146, has been shown to make the approximate shape of an E-block. Unfortunately this is accompanied by porosity, dendritic growth and excessive shrinkage due to the high pouring temperature required to melt both the aluminum and beryllium.
A method for producing a graded metal matrix composite, in U.S. Pat. No. 5,240,672, relies on gravity to settle the reinforcing filler particles of the molten suspension in the bottom of a mold as it slowly cools. This can require a long dwell time to produce the desired composite part.
Another method to produce a graded metal matrix composite, in U.S. Pat. No. 5,525,374, reveals infiltrating metal in a porous ceramic pre-form having varying porosity. Additional expense is incurred because it is a two step process: first the ceramic pre-form is formed, debindered and sintered followed by the second step of metal infiltration while contained in a mold.
U.S. Pat. No. 4,949,194 discloses support arms and support arm assemblies (on E-blocks) formed of ceramic materials such as alumina or silicon carbide. These ceramic materials are conventionally pre-formed into the desired shape, then densified by sintering. Although these ceramic materials offer improvements over the conventional metals, they are far from ideal. For instance, alumina and silicon carbide undergo large dimensional changes from the pre-formed state to the densified state, typically undergoing 18-20% linear shrinkage, depending upon solids volume fraction in the pre-formed state and final sintered density. The large dimensional change makes it difficult to meet component shape and dimension requirements without secondary processing. In addition, alumina, boron carbide and silicon carbide are difficult to machine to the desired dimensions and surface tolerances because of their high hardness and low fracture toughness.
To increase servo bandwidth of the disk drive, to reduce rotational moment of inertia of the actuator and to decrease the gain of comb structural resonances, alternate materials with low density, high specific stiffness and greater damping ratio are desirable.
Consequently, it is the object of this invention to put forth new component materials, and the methods of making same, in order to overcome the problems encountered in the current art while also enhancing the disk drive's performance.